DHCP Services

1. Dynamic Host Control Protocol
(DHCPv4) and for IPv6 (DHCPv6)
Every device that connects to a network
needs a unique IP address
1

2. When an unaddressed client Boots up
2

3. DHCP D.O.R.A Process:-
• DORA between Server & Client
• Discover
• Offer
• Request
• Acknowledge
DHCPv4 message a client sends to accept an
IPv4 address that is offered by a DHCP server
broadcast DHCPREQUEST
Only messages involved BUT clients
Use an gratuitous ARP to check it’s
assigned IP
3

4. Client knows nothing…
Requesting host has no info about DHCP server it
Discovers through this MAC & IP:(broadcast)
FF-FF-FF-FF-FF-FF and 255.255.255.255
a DHCPv4 server sends a DHCPOFFER message to a
client’s MAC address
4

5. DHCP Lease allocation
DYNAMIC = DHCPv4 addr allocation method
assigns IPv4 addrs for a limited lease period
If a client lease is about to expire, client sends a
DHCPREQUEST to original DHCP server
5

6. DHCP from a Router
• Create Pool Name
• Designate Network & Mask
• Designate default gateway for clients =
default-router cmd
• DNS server ( optional)
• excluded-address(es) ( Optional)
• Note lease time & domain name also optional
Cisco IOS DHCPv4 virtual server assigns/manages IPv4 addrs
from specified addr pools within router to DHCPv4 clients
6

7. DHCP setup from a Router
• Create Pool Name = ip dhcp pool rmit
• Designate Network & Mask =
network 10.0.15.0 255.255.255.0
• Default gatway for clients =
default-router 10.0.15.1
• DNS server ip address = dns 10.0.16.1
• Exclude the first 5 addresses =
ip dhcp excluded-address 10.0.15.1 10.0.15.5
To disable DHCP, use the no service dhcp command.
7

8. DHCP on a Dedicated Server
A dedicated DHCPv4 server is scalable - easy to manage
Often networks use enterprise servers located in a
server farm. These servers may provide DHCP, DNS,
TFTP, and FTP services for the network
Often client is on a different LAN than the server & will
need the help of a router to relay it’s discover & ack
messages. When a router forwards addr
assignment/parameter requests, it’s acting as a
DHCPv4 relay agent.
Port 67/68: DHCP/BOOTP client/ server
the ip helper-address command forwards DHCP
messages btw client & Server
8

10. Helper-address is set on the interface
10

11. DHCP request by Routers
To get DHCP automatic addr for a router interface
from a DHCPv4 server issue:
Router()# interface f0/0
Router(-if)# ip address dhcp
11

12. Verification
show running-config | section dhcp output displays the
DHCPv4 cmds configured on R1.
2 pool configured
LAN-POOL-1
LAN-POOL-2
SAME domain!!
12

13. Verification continued
• show ip dhcp binding = list of all IPv4 addrs to
MAC addrs bindings provided by DHCPv4 service.
13
show ip dhcp server statistics,
verify that messages are being
received or sent by the router.
In stateless DHCPv6 configuration, a client
configures its IPv6 address by using the prefix
and prefix length in the RA message, combined
with a self-generated interface ID. It then
contacts a DHCPv6 server for additional
configuration information via an
INFORMATION-REQUEST message. The
DHCPv6 SOLICIT message is used by a client to
locate a DHCPv6 server. The DHCPv6
ADVERTISE message is used by DHCPv6 servers
to indicate their availability for DHCPv6
service. The DHCPv6 REQUEST message is used
by a client, in the stateful DHCPv6
configuration, to request ALL configuration
information from a DHCPv6 server.

14. SLAAC and DHCPv6
DHCPv6 Operations
If stateless or stateful
DHCPv6 is indicated in
the RA message, then
the device begins
DHCPv6 client/server
communications.

15. Stateless DHCPv6
Configuring a Router as a Stateless
DHCPv6 Server

16. Configuring a Router as a Stateless
DHCPv6 Client

17. Verifying Stateless DHCPv6
Verify the stateless DHCP client using the following
commands:
• show ipv6 interface
• debug ipv6 dhcp detail

18.  Explain how DHCPv4 operates in a small- to medium-sized business
network.
 Configure a router as a DHCPv4 server.
 Configure a router as a DHCPv4 client.
 Troubleshoot a DHCP configuration for IPv4 in a switched network.
 Explain the operation of DHCPv6.
Chapter Summary
Summary